1) Field of the Invention
The invention relates to a catalyst based on a zeolite of hexagonal structure related to faujasite and with a high Si:Al ratio. It also relates to the application of the said catalyst to acidic catalysis reactions carried out on hydrocarbon feedstocks.
2) Background Art
Zeolites have many industrial applications. They are employed in particular in the field of adsorbents, detergents or industrial catalysis. In this last field, most of the applications such as catalytic cracking of distillates under vacuum or of residues to petrol or the hydrocracking of distillates under vacuum to diesel fuel are satisfied by catalysts containing a zeolite of the faujasite type, called zeolite Y and having a synthesis Si:Al ratio of between 1.5 and 3.
Thermal and hydrothermal stability of this zeolite and consequently of the catalysts which contain it remains low and the said zeolite must be subjected to post-synthesis treatments to improve the said stability and more generally the performance of the said catalysts. To obtain an increase in the thermal and hydrothermal stability of the said zeolite it is advisable to increase its network Si:Al ratio and for this purpose it is possible to use various treatments, among which there may be mentioned techniques such as steam treatment optionally followed by washing of the aluminium-containing debris with acid (U.S. application Nos. 3,493,519, and 3,506,400), treatment with ethylenediaminetetraacetic acid (G.T. Kerr, J. Phys. Chem., 72 (1968), p. 2594), treatment with SiCl.sub.4 (H.K. Beyer, I.M. Belenykaja, F. Hange, M. Tielen, O.J. Grobet and P.A. Jacobs, J. Chem. Soc., Farad. Trans. 1, 81 (1985), pages 2889 to 2901) or treatment using ammonium fluorosilicate (NH.sub.4).sub.2 SiF.sub.6 (U.S. application No. 4,503,023).
All these treatments are troublesome, and this results in an increase in the cost of the final catalyst. Furthermore, such treatments give rise to structural changes in the treated zeolites, resulting in the appearance of faults which can impair the performance of the catalysts including these treated zeolites.
It has been found that zeolites of hexagonal structure related to faujasite and with a synthesis Si:Al ratio higher than 3 can form the basis of excellent catalysts for cracking, hydrocracking, isomerisation or disproportionation reactions, these zeolites exhibiting, inter alia, a high density of acidic sites of intermediate or high strength and an excellent thermal and hydrothermal stability and being also capable of undergoing, with much fewer inconveniences than zeolite Y, owing to their higher lattice Si:Al ratio, the optional post-synthesis treatments intended, if need be, to increase the said ratio.